Sensitivity of the grassland-forest ecotone in East African open woodland savannah to historical rainfall variation

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Introduction
For East Africa and most other tropical regions where written history extends only to the late-19th century, concrete information on past climate-driven and human-induced landscape dynamics can only be gleaned from natural paleo-environmental archives preserved in lake and bog deposits.Unfortunately, many pollen-based reconstructions of past vegetation change that supposedly indicate early anthropogenic deforestation in East Africa are ambiguous, partly because traditional paleoecological indicators do not unambiguously separate between natural and anthropogenic effects on vegetation with, and partly because of a longstanding bias against the possibility of significant climate-driven vegetation change within the last few millennia (Verschuren, 2004).
Over the past decade, paleoclimate reconstructions with appropriate time resolution have documented strong late-Holocene moisture-balance variation in both eastern and western regions of the East African Plateau (Verschuren et al., 2000;Verschuren, 2001;Stager et al., 2005;Russell andJohnson, 2005, 2007;Russell et al., 2007).Consequently, socio-economic and conservation implications of modern-day human impact on East Africa's ecosystems must now be evaluated against the reference background of a continuously changing, inherently unstable physical environment.Parallel reconstruction of climate (Verschuren et al., 2000) and vegetation history (Lamb et al., 2003) in the central Kenya Rift Valley over the past 1000 years, extracted from one and the same well-dated sediment record, indicated that at least in semiarid eastern portions of the East African Plateau, climate-driven moisture-balance fluctuation at decade-to-century time scales may have caused marked shifts in terrestrial vegetation ecotones; significant anthropogenic forest clearance there seems to have started only in the 17th century AD, after the introduction of maize by Arab merchants.Archaeological and linguistic data from regions north and west of Lake Victoria (Reid, 2000;Robertshaw and Taylor, 2000;Killick, 2009;de Filippo et al., 2012) suggest that (sub)humid western areas of the East African Plateau experienced human landscape modification for a substantially longer period of time.Unfortunately, ambiguity in the pollen records of past vegetation history and/or dating mismatches with the separate proxy records of past climate change have so far hampered efforts to distinguish more clearly climatic and human influences on vegetation history in this region (e.g., Marchant and Taylor, 1998;Taylor et al., 1999;Ssemmanda et al., 2005;Ryves et al., 2011).As a result, it remains difficult to estimate the magnitude of climatic moisture-balance variation responsible for a certain vegetation change inferred from fossil pollen, and to translate pollen assemblage changes into reconstructions of past vegetation change across the landscape.An integrated and quantitative paleoecological approach, including region-specific calibration of paleovegetation proxies (e.g., Vincens et al., 2006;Gelorini et al., 2012b) and their historical validation (e.g., Gelorini et al., 2012a), will be key to a more robust discrimination of vegetation responses to climate change and (pre-) historical human impact.
In this study, we investigate fossil pollen (and fern spore) assemblages in a ∼ 200 year high-resolution lake-sediment record from pristine open woodland savannah in southwestern Uganda to assess the sensitivity of the East African lowland grassland-forest ecotone to historical, decade-scale trends in annual rainfall.Specifically we trace regional vegetation response to three episodes of increased rainfall dated to the 1820s-1830s, ca.1865-1890 and from 1962 to around 2000.Over the nearby Lake Victoria basin, the most recent of these episodes represented a positive rainfall anomaly on the order of 10 % from the long-term mean (Nicholson and Yin, 2001).Analyses of sedimentological characteristics (Bessems et al., 2008) and non-pollen palynomorphs (Gelorini et al., 2012a) from this same record had demonstrated the local occurrence of dry/wet climate cycles which matched the timing of regional climate variability reconstructed from longer lake records nearby (Stager et al., 2005;Russell et al., 2007).In addition, we analysed the modern pollen spectra from surface-sediment samples in eight other small lakes bridging the regional Introduction

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Full 2 Study area and environmental setting 2.1 Setting of the principal study site Lake Chibwera (0 • 09 S, 30 • 09 E) is a medium-sized maar crater lake (76 ha) located at 971 m elevation in the Bunyaruguru lake district of southwestern Uganda, on the floor of the Rift Valley immediately south of Lake George (Bessems et al., 2008) (Fig. 1).The lake is currently fresh, and had a recorded maximum depth of 12.6 m in July 2001 and 12.0 m in August 2008.It is episodically fed by a small creek flowing from nearby Lake Kyamwiga, which in turn receives surface inflow from uplands forming the eastern Rift Valley flank, via the Mbanga stream.Lake Chibwera's own maximum depth is bounded by the sill elevation of a shallow ravine to the north, which during former wetter times may have been an active outlet.Lake Chibwera and its surroundings are situated in the erstwhile Kyambura Game Reserve, which since 1965 forms part of Queen Elizabeth National Park (QENP).The protected Rift Valley landscape of QENP occupies an area of ca.2000 km 2 between lakes Edward and George, and encompasses a wide range of vegetation types (from forest to grassland to papyrus swamp) with a full complement of wildlife including large wild mammal species (buffalo, lion, elephant, waterbuck, Uganda kob; Uganda Wildlife Authority, 2009).Until the early 1970s, the reserve was used for licensed hunting to support the livelihoods of nearby communities.During the period of political instability from the 1970s to early 1990s, uncontrolled hunting throughout the park decimated large-mammal populations, and land in the southern part of the game reserve was occupied by local subsistence farmers and migrants from southern Uganda.Grazing by domestic livestock, mainly goats, inside the reserve was finally halted in 1996, when park management was transferred to the Uganda Wildlife Authority (UWA) (Byaruhanga et al., 2001).Today the largely undisturbed, Introduction

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Regional climate, vegetation and land use
The climate of southwestern Uganda is classified as tropical sub-humid, with a bimodal seasonal rainfall regime peaking in March-May and October-December.Rainfall patterns are strongly driven by monsoonal winds from the Indian Ocean, modulated by the north-south passage of the Intertropical Convergence Zone (ITCZ) across the equator, and associated west-east movement of the Congo Air Boundary (CAB) separating Indian Ocean and Atlantic moisture sources (Nicholson, 1996;Mutai and Ward, 2000).Depending on local topography, mean annual rainfall varies between ∼ 800 and 1300 mm yr −1 ), reaching around 900 mm yr −1 on the Rift Valley floor near Lake Chibwera, and mean annual evaporation varies from ca. 1600 to 1900 mm yr −1 (Bessems et al., 2008).As indicated by historical gauge measurements and water-level reconstructions from Lake Victoria (Piper et al., 1986;Nicholson and Yin, 2001;Stager et al., 2005), regional rainfall has fluctuated substantially in the past ∼ 200 years, with periods of relative drought (e.g., in the late 18th/early 19th century and AD ∼ 1920-1962) alternating with episodes of wetter conditions (e.g., ∼ 1865-1885 and ∼ 1962-2000; Fig. 2).
Modern-day vegetation on the uplands of the Rift Valley escarpment east of our study area is identified as Lake Victoria Regional Mosaic (White, 1983), which is a forest/savannah mosaic with remnants of evergreen or semi-deciduous forest 1680 Introduction

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Full (specifically the Kasyoha-Kitomi Forest Reserve), and savanna woodland with trees such as Acacia, Combretum and Euphorbia.Here, persistence of the understory grass layer (mainly Pennisetum purpureum and Themedra triandra) depends on regular burning, either natural or (increasingly) anthropogenic.Demographic pressure has significantly reduced Uganda's natural forests and bushlands (Andrua, 2002); except for national parks, forest and wildlife reserves, most landscapes in the crater lake-districts are now strongly impacted by agricultural activity (cropland and plantations), in our study area to within 2 km of Lake Chibwera (Fig. 1).Brown, 1964;Vincens et al., 1997;Kirabo et al., 2011).Montane rainforest (e.g., Podocarpus milandjianus, Rapanea melanophloeos, Prunus africana) and Afroalpine vegetation (Ericaceae) is mostly restricted to the Rwenzori Mountains (Vincens et al., 1997), over 50 km to the northwest of Lake Chibwera.

Sediment sampling, lithology and chronology
Sediment core CHIB02-1P (0-117 cm) was recovered in two sections from the deepest part of Lake Chibwera, using a rod-operated single-drive piston corer (Wright, 1980).
The unconsolidated uppermost sediments (∼ 25-30 cm) were extruded upright in 1 cm increments with a fixed-interval sectioning device (Verschuren, 1993), and transferred to Whirl-Pack bags.The deeper, more consolidated sediments were retained intact Introduction

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Full in the original core tubes (Bessems et al., 2008).Due to depletion of uppermost mud following previous analyses, we extended the Chibwera sequence upward 9 cm (now totaling 126 cm), using the overlapping UWITEC gravity core CHIB08-1G collected at the same GPS position (Gelorini et al., 2012a).The cores were crosscorrelated using data of bulk sediment composition (based on loss-on-ignition, LOI) and magnetic susceptibility, and the depths of lithostrigraphical transitions defined by Bessems et al. (2008)  The extended composite sequence of Lake Chibwera consists of four lithostratigraphic units (Fig. 4).Unit 1 (126-121 cm) is composed of dry but organic clays with few large plant fragments, representing a poorly defined pre-19th century lacustrine phase (Bessems et al., 2008).It is overlain by a horizon of dry clay (Unit 2, 121-112 cm) with modest organic content (11-16 %), marking the lake's complete desiccation (but likely seasonal flooding) during a prolonged episode of extreme regional drought in the late 18th to early 19th century (Verschuren, 2004;Bessems et al., 2008).This desiccation horizon is overlain by a section of moist, organic peaty clay with abundant coarse plant fragments (Unit 3, 112-101 cm), representing the early to mid-19th century phase of gradual lake filling, characterized by a shallow-water environment with submerged macrophytes and swampy riparian zone.It is followed by soft organic clays (Unit 4, 101-0 cm), reflecting the true lacustrine phase with mostly higher lake level which has prevailed from the 1860s until today (Bessems et al., 2008).
The relationship between fossil pollen assemblages and actual vegetation cover across the grassland-forest ecotone of southwestern Uganda was calibrated using modern-day pollen spectra from nine surface-sediment samples recovered from Lake savannah woodland (Chibwera) or bushland (Bagusa, immediately south of Lake George) to moist semi-deciduous forest (Kacuba, Kyasanduka and Nyamusingire, all in Maramagambo forest) (Fig. 1).The two other lakes (Kanyamukali and Katinda) were originally located near the woodland-forest border but are now strongly impacted by land clearance and agricultural activity.

Pollen analysis
From the Chibwera sediment sequence, in total 33 samples of 1 mL volume were extracted for pollen analysis, mostly from 1 cm increments at 4 cm intervals (except for two 3 cm increments in the uncompacted upper 10 cm).In all other lakes, pollen was extracted from homogenized samples of the upper 3-5 cm of uncompacted surface muds, which according to 210 Pb-dating results (Russell et al., 2007;Bessems et al., 2008) on longer sequences from four of these lakes (Chibwera, Katinda, Kanyamukali, Kitagata) have accumulated in the last four to ten years.Following standard procedures (Faegri et al., 1989), the samples were treated with warm 10 % KOH, warm 10 % Na pyrophosphate, 96 % acetic acid, an acetolysis mixture of 1 : 9 H 2 SO 4 and acetic anhydride boiled to 100 • C for 10 , and finally 96 % ethanol.A bromoformethanol mixture with specific gravity 2.0 was used to separate pollen grains from heavier mineral sediment components.The microfossil extracts were then put in glycerine, dried overnight in an oven at 40 • C and finally mounted on microscope slides.
Pollen (and fern/moss spore) nomenclature and identifications in this study are based on the African Pollen Database (APD) (for details: http://medias.obs-mip.fr/apd; Vincens et al., 2007).In most samples at least 500 terrestrial pollen grains were counted (range 275-1313; mean 638).If Poaceae pollen counts were too low to ensure a solid percent abundance estimate, counting continued until ideally at least 300 Poaceae pollen grains were encountered.Abundances of individual pollen and spore taxa are expressed as percentages of the non-local pollen sum (i.e., excluding (semi-) aquatic taxa such as the Cyperaceae, and ferns and mosses); the results were plotted using TILIA 1.7.16 (Grimm, 2011).Pollen assemblage zones Introduction

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Full (PAZ) were generated using stratigraphically-constrained incremental sum-of-squares cluster analysis (CONISS, Grimm, 1987), applied to the stratigraphic distribution of all terrestrial pollen taxa.

Modern pollen spectra
Modern pollen spectra of the nine study lakes sorted by biome (Fig. 3) highlight the descending relative abundance of Poaceae pollen across the natural grassland-forest ecotone, from ∼ 65 % in grass savanna with dispersed trees and shrubs (Kitagata and Kikorongo) to ∼ 50 % in open savannah woodland and bushland (Bagusa and Chibwera) and ∼ 35 % in moist semi-deciduous forest (Nyamusingire, Kacuba and Kyasunduka).Poaceae abundance at the impacted lakes Kanyamukali and Katinda is similar to those of open savannah woodland, despite their position close to the original woodland-forest boundary.Removal of trees and shrubs for agricultural purposes in the vicinity of these lakes created pollen assemblages that are seemingly derived from vegetation growing under drier climatic conditions (cf.Vincens et al., 2003).
Assemblages from the anthropogenically impacted sites Katinda and Kanyamukali are readily distinguished by high percentages of Asteraceae (5.4 and 14.5 %, respectively) and the cultural indicator Ricinus communis (3.6 and 7.0 %).Pollen from the exotic Eucalyptus, introduced in Uganda from the mid-20th century onwards (Langdale-Brown, 1964), is common (8 % of the pollen sum) in Lake Kanyamukali, but in Lake Katinda (ca. 2 %) it is not more common than at the three semi-deciduous forest sites and Lake Bagusa, which are all located inside QENP but within a few km of densely populated areas.These values are consistent with a land-use/land-cover survey in 25 Ugandan crater basins (Gelorini et al., 2012a), which recorded Eucalyptus plantation occupying ∼ 15 % of the Kanyamukali crater catchment but lacking at Lake Katinda.Introduction

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Full abundance in our surface-sediment samples is likely inversely proportional to distance from its nearest important pollen source in farmlands to the east, southeast or north of the study area.
The case of Eucalyptus highlights the mosaic distribution of natural and impacted vegetation types in our study area at the spatial scale of individual sites' relevant pollen source area, resulting in "mixed" pollen assemblages (Vincens et al., 2006).This also explains the presence of typical savannah trees (Acacia, Maerua, Euclea) in our pollen spectra from inside Maramagambo forest; although park vegetation immediately surrounding all three lakes may be relatively pristine, farmland or degraded forest occurs to within a few km of these sites (Fig. 1).The percent Poaceae pollen recorded at our three forest sites nevertheless remains markedly lower than at our open savannah woodland sites (Fig. 3).Similarly, the presence of Acalypha (here most likely the low-stature woodland shrubs A. fruticosa and A. indica), Phoenix reclinatatype and Celtis in pollen spectra from pristine grass savannah sites can be considered "contamination" from more densely wooded savannah and/or strips of riverine forest nearby, or from more distant semi-deciduous forest.Nevertheless, the percent grass pollen recorded at the true grass savannah sites remains markedly higher than at wooded savannah sites (Fig. 3).
The apparent over-representation of Poaceae at our three forest sites is consistent with the results of Duffin and Bunting (2008), who in the mosaic vegetation of a southern African savanna found that local grasses produced about twice as much pollen as the arboreal taxa.Nevertheless, in our study region the summed pollen percentage of true forest trees increases from 5.1-13.8% in grass and woodland savannah sites to 12.5-20.9% at the three sites inside semi-deciduous Maramagambo capensis-type (likely Olea welwitschii) and Allophylus are associated with evergreen and moist semi-deciduous forest (Vincens et al., 2006), and here most likely originates from the Kasyoha-Kitomi Forest Reserve, situated at slightly higher elevations along the eastern Rift Valley escarpment to the south-east of our study area.This is also the likely source of Dodonaea viscosa-type and Alchornea pollen, given their rather indifferent values across our nine calibration sites.The decreasing trend in percent Poaceae from grass savannah to wooded savannah and semi-deciduous forest is mostly compensated by local/regional trees and shrubs (Fig. 3).Savannah woodland sites (except the impacted Kanyamukali) show elevated percentages of Acalypha and/or Phoenix reclinata-type, whereas Maramagambo forest sites show elevated percentages of Celtis and Flueggea virosa, besides less pronounced increases in Acalypha and Phoenix.Acacia and Rhus type vulgaris show an opposing trend, with slightly more robust appearances in some grass and wooded savannah sites than in Maramagambo forest sites.The common savannah woodland tree Euclea displays highly variable occurrences.Among herbaceous taxa, percentage values of Urticaceae are markedly higher in the two wooded biomes than in open grass savannah, consistent with its preference for shaded places at forest edges, in wooded grassland, or along streams (Friis, 1989).In summary, pollen assemblages from the region's modern-day open grass savannah can be distinguished by ∼ 65 % Poaceae accompanied by Acacia and Rhus type vulgaris, characteristic for plant functional type tr3+g in the East African SAVA (savannah) biome (Vincens et al., 2006)

Pollen stratigraphy of the Chibwera sediment sequence
CONISS stratigraphic zonation of fossil pollen assemblages in the Chibwera sediment sequence defines four major pollen assemblage zones (Chib-1 to Chib-4), of which the latter three are each subdivided in two distinct subzones (Fig. 4).Zone Chib-1 (126-111 cm, 18th century) is stratigraphically related to the dry, mostly low-organic clays which were deposited when Lake Chibwera stood low or (at least seasonally) dry (Units 1-2); consequently, the four assemblages comprising this basal zone may not necessarily represent an uninterrupted sequence.They are characterized by very high Poaceae abundances (75-85 %) and the robust presence of Asteraceae, Chenopodiaceae and Amaranthaceae (Achyranthes-type aspera), with minor contributions of Ipomoea-type and Vernonia-type schimperi.Pollen from Podocarpus, Allophylus and Olea represents long-distance dispersal from either the Rwenzori or the eastern Rift Valley escarpment.Pollen from woodland trees and shrubs is scarce, with only small and often occasional occurrences of Acacia, Rhus type vulgaris and Flueggea virosa.The overall terrestrial pollen assemblage suggests that vegetation surrounding Lake Chibwera was a dry grass savannah with less tree cover than modern-day savannah at any of our calibration sites.The abundant Cyperaceae pollen (40-60 %) in this zone, supplemented with Typha domingensis-type, suggests that much of the seasonally flooded dry lake floor was occupied by a sedge swamp with cattails, and mosses and ferns (monolete and trilete spores; Fig. 4) growing at their base.Together, the pollen evidence implies a climatic moisture balance markedly more negative than today, consistent with the lithostratigraphy (Bessems et al., 2008).
Sub-zone Chib-2a (111-84 cm, ca.1820-1895) corresponds with the section of organic peaty clay (Unit 3) deposited under the shallow and swampy, but permanent lacustrine conditions during the first half of the 19th century, and the lower part of organic lake muds deposited under shallow to deep lacustrine conditions from 1687 Introduction

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Full Alchornea, mostly rare before, is now consistently present.Among the herbaceous taxa, the Urticaceae do well but also Asteraceae, Cyatula-type orthocantha, Solanum-type, Swertia usambarensis-type and Tarenna-type graveolens; in contrast, Chenopodiaceae disappear almost completely.Within the group of aquatic taxa, Cyperaceae retreats from its earlier maximum, although not in synchrony with the reduction in Poaceae.Overall this pollen sub-zone suggests the presence of savannah woodland with greater tree cover than today, and more damp soil habitat promoting Phoenix reclinata.The increases in Allophylus and Macaranga-type trees suggest a simultaneous expansion of nearby evergreen and moist semi-deciduous forest, all indicative of climatic conditions significantly wetter than today.
Sub-zone Chib-3a (67-31 cm, ca. 1925Chib-3a (67-31 cm, ca. -1970) ) is characterized by the strong rebound of Poaceae pollen to 70-75 %, whereas woodland trees such as Myrica, Celtis, Flueggia virosa, Acalypha and Phoenix reclinata-type, as well as all forest trees, experience modest to strong reductions.Among the herbaceous taxa, Asteraceae and Cyatula-type orthacantha of the Amaranthaceae almost disappear, and also the presence of Urticaceae is less robust than throughout Zone Chib-2.The Chenopodiaceae again expand in synchrony with the Poaceae, supplemented byCommelina-typesbenghalensis and forskalaei, and Justicia-type odora (the latter for the first time consistently present).The Cyperaceae initially maintain their starting value of ca.20 %, but then somewhat unexpectedly fall to values of ca. 10 % in the upper half of this sub-zone.Overall, this pollen sub-zone documents the return of a more negative climatic moisture balance, and the resulting decline of tree cover in the savannah woodland surrounding Lake Chibwera.
The three pollen assemblages defining Sub-zone Chib-3b (31-21 cm, ca.1970 to the late 1980s) are broadly similar to those of Chib-3a, except for increases in Allophylus, Myrica, Celtis, the Urticaceae and Tarenna-type graveolens, a robust re-appearance of Alchornea, and the (near-) disappearance of Chenopodiaceae, Commelina-typebenghalensis and Justicia-type odora.These species responses suggest the return of a wetter climate, however the percent abundances of Acalypha Introduction

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Full   4), along with now solid presence of the cultural indicator Ricinus communis (which had its first appearance in Sub-zone Chib-3b).Also the return of Asteraceae is now likely promoted by the intensifying land use for subsistence farming outside (and encroaching into) QENP.Sub-zone Chib-4a (21-6 cm, late 1980s to ca. 2004) groups four samples with greatly reduced Poaceae values (45-50 %), modest to large increases in Celtis, Acalypha and Phoenix reclinatatype, and relatively strong showings of Macaranga-type, Myrica, Euclea, Alchornea, and Combretaceae/Melastomataceae.Also notable is the re-appearance of Acacia, Rhus type vulgaris and Ficus, with modest values but nevertheless a more prominent occurrence than at any time since the early 19th century.Among the herbaceaous taxa we note the high abundances of Urticaceae, and continuous presence of Swertia usambarensis.Among the aquatic taxa, the very low values of Cyperaceae pollen (10 %) indicates that a highstand of Lake Chibwera flooded most of the fringing sedge swamp which previously must have existed inside its crater basin.Typha domingensis-type seems to have thrived better than before, unless its pollen was brought to Chibwera from cattail stands along the inflowing stream.This pollen subzone documents the now full-scale response of regional vegetation to a more positive climatic moisture balance, initiated in Sub-zone Chib-3b.Given the limited effect of all newly appearing anthropogenic plant taxa (not locally, but within the pollen source area) on the percent abundance of Poaceae pollen (together 4-5 %), this pollen zone documents the third episode of expanding tree cover in the savannah woodland surrounding Lake Chibwera in the past 200 years.

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Full to 3 %) in pollen zone  indicate that these plants expanded on and around the seasonally flooded Chibwera lake bed during the late 18th-century drought, possibly benefiting from soil disturbance caused by large mammals.The three inferred wet phases (ca. 1820-1850, ca. 1880-1920, ca. 1980-2004) are characterized by low (45-50 %) or decreasing percentages of Poaceae pollen, and at least modest increases in savannah trees and shrubs (Acacia, Rhus type vulgaris) and herbs such as Asteraceae, Urticaceae, Solanum and Swertia usumbarensis-type.Also taxa of denser woodland and shrub savannah (Acalypha, Myrica, Combretum), riparian forest (Phoenix reclinata-type) and moist or semi-deciduous forest (Celtis, Macarangatype, Alchornea, Allophylus) show a repeated positive response during these episodes.
Clear peaks in (otherwise rare) Ericaceae pollen during the latter two of these periods supports our earlier suggestion that this is pollen may originate from low-elevation heather species occurring along the Rift Valley escarpment, rather than the distant Afroalpine zone in the Rwenzori.The strongest responses (i.e., with greatest impact on the relative abundance of Poaceae) occur in Acalypha and Phoenix reclinata-type.Some savanna and woodland trees, such as Euclea and Flueggea virosa, resemble the latter two taxa by expanding strongly at the end of the lake drystand ca.AD 1820 , but then show fairly stable (Flueggea) or gradually increasing (Euclea) abundances over the following 200 years.Trends in Asteraceae are mostly opposite to those in Poaceae, except that both peak during the late 18th-century drought.Even allowing for some promotion of Asteraceae by intensifying human activity along park boundaries since the 1980s (Fig. 4: zone Chib-4a), it appears that in this open woodland savannah, Asteraceae tend to benefit from increased moisture while also having taken advantage of new habitat created by a lake drystand.
The inferred alternation of wet and dry episodes in the Chibwera region over the past ∼ 250 years is consistent with historical data and independent lake-based reconstructions of East African climate change within recent centuries (Verschuren, 2004).Severe late-18th century drought (lasting until AD 1815-1820) followed by high rainfall in the 1820s has now been documented as a cycle of desiccation and refilling in

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Full shallow lake basins throughout equatorial East Africa (e.g., Verschuren, 1999;Russell et al., 2007;Bessems et al., 2008).Compilation of historical records indicates that the region's climate was again relatively dry from the 1830s to the early 1850s (Nicholson and Yin, 2001).Lake Victoria is said to have risen rather steadily in the first decades after its discovery in 1858, until reaching its highest documented level in 1878.This was followed by a rapid decline in the 1880s, and relative stabilization around its presentday level from the 1890s until 1961-1962, when exceptional rainfall caused high Nile discharge and lake transgressions across East Africa (Flohn, 1987).Notwithstanding intermittent dry years in the 1970s and 1980s, this somewhat more positive regional moisture balance has been maintained for four decades; only within the last decade has Lake Victoria regressed to levels approaching its pre-1961 mean (Fig. 2).Also the level of Lake Chibwera fell 0.6 m between our field campaigns in July 2001 and August 2008; the accompanying expansion of riparian wetland likely explains the more substantial presence of Cyperaceae pollen in the uppermost cored sediments (zone Chib-4b).The lake-level reconstruction for Lake Victoria by Stager et al. (2005), which is based on the percent abundance of shallow-water diatoms in the sediment record of one of its peripheral bays, confirms that in the late 18th and earliest 19th centuries, also Lake Victoria stood lower than at any time since then (Fig. 2).
Strong congruence of the temporal pattern of pollen-inferred vegetation change around Lake Chibwera with this history of regional climate change suggests a causal relationship.However, the exact timing of reconstructed vegetation changes appears somewhat delayed relative to the improving or deteriorating moisture balance which is likely to have caused them (compare Fig. 4 with Fig. 2).The discrepancy can be attributed partly to dating uncertainty in the Chibwera sediment record (see Bessems et al., 2008) and Urticaceae expanded rapidly (already at the base of zone Chib-3b), followed by Solanum-type and Swertia-type, and finally by Acalypha and Phoenix reclinata-type; it is the latter which drive much of the recorded decline in Poaceae pollen.

Sources of pollen in Lake Chibwera sediments
Lake Chibwera occupies a low-rimmed maar crater basin, situated in an open grassdominated landscape grading into woodland and forest occupying the uplands which flank the Rift Valley to the north and south.As a result, both today and in the past Lake Chibwera has captured pollen from a variety of biomes: Afromontane forest, closed-canopy lowland evergreen and semi-deciduous forests, open-canopy savannah woodland and grass and shrub savannah.The pollen record of vegetation change recorded in Lake Chibwera sediments therefore reflects vegetation response to climate variability both in the immediately surrounding landscape (excluding truly local (semi-) aquatic vegetation) and at a broader regional scale (cf.Jackson, 1990;Sugita, 2013).Discerning between them is facilitated by comparisons with the composition of pollen spectra deposited at reference sites today (Mathias and Giesecke, 2014).In this study, these are distributed across the regional forest-grassland ecotone which in turn reflects a (largely topographic) gradient in climatic moisture balance.Also the relevant pollen source areas of each reference site, which for "medium-sized" lakes in these (semi-) open forests, woodlands and grasslands are on the order of 600-900 m in diameter (Duffin and Bunting, 2008;Suguita, 2013), all encompass multiple types of vegetation, resulting in mixed assemblages (cf.Sect.4.1).Nevertheless, systematic differences in the percent abundance of characteristic taxa associated with each biome help to assess the magnitude of vegetation change which occurred at Lake Chibwera in the last ∼ 250 years.
The largest fraction of pollen in both modern-day and fossil assemblages preserved in the sediments of small and medium-sized lakes originates from vegetation in their near surroundings (Jacobson and Bradshaw, 1981;Broström et al., 2004;Duffin and Bunting, 2004), and are delivered to the lakes either by wind, precipitation or local Introduction

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Full surface run-off (Moore et al., 1991).This includes the typically airborne pollen of Poaceae, Euclea, Capparidaceae species, Flueggea virosa, Rhus type vulgaris and other taxa common to the local SAVA biome, and of some trees in the forest of Kyambura gorge ∼ 5 km to the west.Pollen of Afromontane Podocarpus and Prunus africana-type must be advected exclusively by wind, from the Ruwenzori mountains.
An unknown but sizable fraction of pollen from Celtis, Macaranga, Allophylus and Dodonaea viscosa-type, which grow in either pristine or degraded fragments of moist and semi-deciduous forest on the eastern Rift Valley flank, reaches Lake Chibwera by surface inflow via the Mbanga stream and Lake Kyanwiga.This may also be the case for Cupressaceae and Eucalyptus, plantations of which now partly replace this forest.
In the case of Cupressaceae, however, an alternative pollen source are cypress trees planted in the 1980s at a camping site in the direct vicinity of Lake Chibwera (Gelorini et al., 2012a).Advection by wind from intact forest further to the east (Kasyoha-Kitomi) or south (Maramagambo) likely contributes a lesser fraction of the forest-tree pollen reaching Lake Chibwera, considering that the distance to these sources mostly exceeds 10 km.The principal source of Phoenix reclinata-type and Acalypha pollen, the two taxa showing the strongest response to climate-driven moisture-balance change in the Chibwera record, is uncertain.Phoenix reclinata is a characteristic component of local stands of riparian forest, hence stream inflow from distant stands is potentially a major contributor.However, riparian forest also occurs in shallow ravines traversing the savannah to the north and west of Lake Chibwera (Fig. 1), and it occupies sizable areas of damp soil in dry crater basins immediately to the south and east.Thus, increasing abundance of Phoenix reclinata-type pollen may reflect a true expansion of riparian forest and damp soil habitat in the near vicinity of Lake Chibwera, with deposition of wind-dispersed pollen.Similarly, stream inflow is a likely source of pollen from Acalypha herbs and shrubs growing along distant forest margins, but Acalypha fruticosa is a shrub of dry woodlands and shrub savanna (Hemp, 2006), such as now occurs to the north of Lake Chibwera towards Lake George (Fig. 1).Crucially, for both Introduction

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Full Acalypha and Phoenix, the fraction of pollen contributed by stream inflow may vary together with climatic moisture balance, such that the effective size of Lake Chibwera's pollen source area for these taxa cannot be assumed constant through time.A wetter climate is likely to both promote populations near Lake Chibwera and enhance the input of pollen from more distant (potentially stable) stands.

Vegetation history of open woodland savannah in the Rift Valley of western Uganda
Throughout the ∼ 250 year time window represented by the studied sediment sequence, Lake Chibwera has been situated in vegetation of plant functional type tr3+g belonging to the SAVA biome (Vincens et al., 2006).This is indicated by the continuously high (variable, but always > 45 %) percentages of Poaceae pollen, the low combined percentage of trees and shrubs with a Sudanian (White, 1983) phytogeographical affinity such as Euclea, Maerua and Acacia, and the consistent presence of dryland herbs belonging to the Amaranthaceae, Chenopodiaceae and Commelinaceae.Dry phases in the Chibwera pollen record display higher Poaceae percentages (maxima of 75-80 %) than recorded in the modern-day samples from our two grass savannah sites, in which they account for ca.63-67 % of the pollen sum.This implies that during historically dry episodes within the past ∼ 200 years, treeless grassland was more extensive in the landscape around Lake Chibwera than is the case today at QENP sites located more centrally in the dry Rift Valley between lakes Edward and George (Fig. 1).Even less woodland trees and shrubs were present during the late-18th century drought when Lake Chibwera stood dry, but Poaceae pollen did not attain higher percentage values because of peak contributions from herbs (Asteraceae, Achyranthes, chenopods) growing on and around the dry lake bed.
In the pollen assemblage of the earliest wet episode, dated to the mid- 20th century, display grass pollen values that are substantially lower still (45-50 %), approaching that of our reference sites located in protected semi-deciduous forest.
It is doubtful that during these wet periods the entire landscape around Chibwera developed high tree cover, or that regional forest cover contracted during dry periods.
Excluding Acalypha and Phoenix reclinata-type, the combined abundance of forest tree taxa remains rather low, and many taxa are found only occasionally.Together, this implies a distant location of the moist and semi-deciduous forests from which taxa such as Macaranga, Celtis, Olea capensis-type, Allophylus, Holoptelea grandis, Myrisine africana, Cassia-type, Embelia and Diospyros originate, throughout the period represented by this record.More limited stream discharge from the eastern escarpment during dry episodes further reduced the visibility of these taxa in the Chibwera pollen assemblages.The comparatively higher percent abundances and strong apparent climate response of Acalypha and Phoenix reclinata indicate that multi-decadal variation in climatic moisture balance caused significant changes in the regional prevalence of savannah woodland/bushland and riparian forest stands, beyond what can be explained by changes in stream input.

Conclusions
Combination of a high-resolution pollen record with modern pollen-assemblage data from reference sites spanning the modern-day regional vegetation ecotone permitted Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | grassland-forest ecotone, to more accurately reconstruct climate-driven vegetation change across the landscape through time.
Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Chibwera and eight neighbouring crater lakes.Seven of these lakes are located inside the protected area of QENP, and span the vegetation gradient from open grass savannah (Kikorongo and Kitagata, both north of Kazinga Channel) to open Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | forest.Pollen from Podocarpus and Prunus africana-type originate from Afromontane vegetation on the Ruwenzori Mountains, at least 30 km away.Also the modest amount of Ericaceae pollen recovered at our reference lakes likely originates in the Rwenzori, although the occurrence of short-stature Ericaceae on the local Rift Valley escarpment cannot be excluded (Henry Osmaston, personal communication).Pollen from Olea Introduction Discussion Paper | Discussion Paper | Discussion Paper | . Other typical SAVA trees, however, are here associated with degraded portions of the semi-deciduous forest (Maerua of the Capparidaceae and Flueggea virosa) or display an erratic distribution across sites (Euclea).Pollen assemblages of savannah woodland is distinguished by ∼ 50 % Poaceae with strong contributions of Acalypha and/orPhoenix reclinata-type, and further Olea and Combretaceae/Melastomataceae.Pollen assemblages from semideciduous forest is distinguished by ∼ 35 % Poaceae and diverse arboreal taxa of plant functional type tr1 in the East African TSFO (tropical seasonal forest) biome (Vincens Discussion Paper | Discussion Paper | Discussion Paper | et al., 2006), such as Celtis, Holoptelea grandis, Cassia-type, Macaranga-type, Olea and Allophylus.
Discussion Paper | Discussion Paper | Discussion Paper |about 1860 onwards (Unit 4).In the temporal sequence of pollen assemblages, three successive phases can be discerned.At the base of this subzone marked decreases of Poaceae (to 55-60 %), Asteraceae, Chenopodiaceae and Achyranthestype aspera are compensated by a temporary expansion of Acacia and Rhus type vulgaris, along with the herbs Polygonum senegalense-type and Solanum-type.Myrica and the Urticaceae record similarly rapid but more long-term increases.Shortly thereafter the summed abundance and diversity of woodland tree taxa increases strongly, most notably by Acalypha and Phoenix reclinata-type but also Ficus, Myrica, Celtis, Flueggea virosa, Euclea, Baphia-type and a representative of the Combretaceae/Melastomataceae.Together with the robust presence of Urticaceae, this diversification of tree taxa and reduction in Poaceae suggests wetter climatic conditions promoting the development of a type of savannah woodland not unlike the vegetation surrounding Lake Chibwera today (compare with Fig.3).The strong reductions in Cyperaceae (to 25 %), Typha domingensis-type and undifferentiated fern and moss spores reflect lake filling, in which emergent aquatic vegetation retreated to a more or less narrow belt along the shoreline.However, in the upper half of this subzone (dated between 1850 and 1895), Poaceae pollen abundance increases again to 70-80 %, along with a renewed increase of Chenopodiaceae, and appearances of other dryland herbs such as Commelina-type forskalaei, Tarenna-type graveolens and Justicia-type odora.Expansion of these taxa is mostly at the cost of Acalypha, but also of Ficus, Myrica, Euclea and Baphia-type along with Solanum-type herbs.The simultaneous increase in Cyperaceae pollen (peaking at 35 %) suggests an expansion of the littoral sedge swamp and thus a lowering of lake level, but this is not clearly recorded in the lithostratigraphy.Pollen assemblages of sub-zoneChib-2b (84-67 cm, ca.1895Chib-2b (84-67 cm, ca.-1925) )  are characterised by a strong decline in Poaceae pollen to its lowest level (45 %) of the entire sequence, accompanied by strong rises in woodland trees such as Acalypha, Phoenix reclinata-type, Celtis, Flueggea virosa and Euclea, but also true forest trees such asAllophylus, Macaranga-type and Dodonaea viscosa-type.Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 50 %) and reductions in, amongst others, Celtis, Acalypha, Macaranga, Allophylus, Combretaceae/Melastomataceae, and Swertia usambarensis.Woodland taxa such as Phoenix, Euclea and Alchornea remain stable, creating uncertainty as to whether increasing Poaceae pollen actually reflects vegetation response to a new trend of climatic drying.The recent reductions in influx of Allophylus and Celtis pollen may also result from human encroachment into the Kasyoha-Kitomi forest.The only robust indication in the Chibwera pollen record of a recent drying trend is the doubling of Cyperaceae pollen in sub-zone Chib-4b compared to Chib-4a, which we interpret to reflect expansion of the local sedge swamp, and thus a lowered lake level.-assemblage signatures of vegetation response to climate change Alternating opposite trends in the percent abundances of grass (Poaceae) pollen and some arboreal taxa, particularly Acalypha and Phoenix reclinata-type, in the Lake Chibwera record define a historical sequence of six distinct episodes of climatedriven vegetation change in the past ∼ 250 years.To a large extent, stratigraphic zone boundaries determined by numerical clustering of successive pollen assemblages (Fig. 4: CONISS zones Chib-1 to Chib-4b) coincide with the levels where shifts (increases/decreases) in the Poaceae percentage curve become apparent.Three episodes of inferred dry climatic conditions (late 18th century to ca. 1820, ca.
Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | . Still, the late 20th-century vegetation response likely caused by the improved moisture balance which started in the early 1960s is recorded in sediments dated to the 1970s and up to around 1980 (the Chib-3b/Chib-4a zone boundary), i.e. well above the robust 137 Cs marker horizon of peak atomic bomb testing in 1964 (at 36 cm; Fig. 5).Taxa such as Allophylus, Myrica, Rhus type vulgaris, Celtis, Alchornea Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 19th century, Poaceae drops to percentage values of ca.55 %, equivalent to those of Lake Chibwera and other open woodland savannah sites within QENP today.The two other recognized wet episodes, dated to the late 19th-early 20th century and the last decades of the Discussion Paper | Discussion Paper | Discussion Paper | tracing local vegetation response to (multi-)decadal climatic moisture-balance variation in a natural savannah landscape of western Uganda.We find that the landscape surrounding Lake Chibwera in Queen Elisabeth National Park has been an open woodland savannah throughout the past 200 years, with historical rainfall variability exerting relatively modest effects on local tree cover (mostly the abundance of Acacia and Ficus) and the prevalence of damp soil areas (reflected in the abundance of Phoenix).Enhanced pollen influx via upland streams seems to be responsible for the strong apparent expansion of true forest trees during wet episodes.Hence, isolation of Discussion Paper | Discussion Paper | Discussion Paper | the truly local vegetation response will require palynological investigation of a savanna site with similarly continuous deposition but limited hydrological sensitivity compared to Lake Chibwera.On the positive side, the (site-specific) influence of pollen from exotic trees and other cultural indicators fails to mask the region's natural vegetation dynamicsDiscussion Paper | Discussion Paper | Discussion Paper |

Fig. 1 .
Fig. 1.Map of modern-day vegetation distribution in the Bunyaruguru area of southwestern Uganda (equatorial East Africa, cf.inset map), showing the locations of Lake Chibwera and eight pollen reference sites within and near Queen Elisabeth National Park (red dash-stippled line is the park boundary).Modified from the National Biomass Study (1996) map of Uganda land cover stratification.

Fig. 1 .
Fig. 1.Map of modern-day vegetation distribution in the Bunyaruguru area of southwestern Uganda (equatorial East Africa, cf.inset map), showing the locations of Lake Chibwera and eight pollen reference sites within and near Queen Elisabeth National Park (red dash-stippled line is the park boundary).Modified from the National Biomass Study (1996) map of Uganda land cover stratification.

Fig. 3 .
Fig. 3. Percentage abundances of selected terrestrial pollen taxa recovered from surficial bottom sediment of nine reference lakes in the Bunyaruguru area.The selection is identical to that shown in Fig.4, except for seven types (Baphia type, Embelia type, Hymenodictyontype floribundum, Sericostachys-type scandens, Polygonum senegalense-type, Commelinatype forskalaei and Justicia-type odora) not found in these surface-sediment samples.Sites are ordered from top to bottom according to their position along the regional grassland-forest gradient.
natural landscape around Lake Chibwera is a wooded grassland, composed mainly of the grasses Themeda triandra, Imperata cylindrica and Cymbopognon nardus with dispersed Acacia gerrardii, Acacia siberiana, Capparis tomentosa and Ficus sp.Bush thickets (evergreen bushland; White, 1983) of Capparis tomentosa and Euphorbia candelabrum trees shelter Jasmina sp. and Ipomoea sp.; Sporolobus pyramidalis is a common grass.Riparian forest in nearby Kyambura Gorge and along the ravines of ephemeral streams include trees such as Cynometra sp. and Diospyros abyssinica, and the palm Phoenix reclinata in wet open-canopy areas (Krueger Maesopsis eminii, Olea welwitschii, Sapium ellipticum and Phyllanthus discoideus characterize the colonizing stage, followed by a succession of mainly Khaya anthotheca, Entandrophragma ssp.and Cynometra alexandri, and climax vegetation composed of Cynometra alexandri with Celtis ssp.and Strychnos mitis (Langdale- and Phoenix reclinata-type remain modest, and Poaceae values remain high at 65-75 %, i.e. similar to values in the grass savanna with limited tree cover near lakes Kitagata and Kikorongo today.Pollen Zone Chib-4 (21-0 cm, late 1980s to 2008) is characterized by the appearance of exotic cypress and eucalyptus trees in the Chibwera sediment record (Cupressaceae and Eucalyptus in Fig.